Make "Pop Up" Plugin responsive with page?

Hi,

I noticed that the "pop up" plugin appears on a certain part of the page - as indicated in the settings. How do I make it so that it appears wherever the user is on the page? For example: if the user is scrolling down the article, I want the pop-up to appear wherever he is in the center of the screen. So if he is reading a post, and he is half-way down the screen, the pop-up should appear .

Thank you